1. Field of the Invention
This invention relates to the preparation of oxygenate-containing finished gasoline, wherein the finished gasoline is manufactured by mixing an oxygenate-free substantially hydrocarbon blend, also herein referred to as “xBOB”, with a known, constant quantity and constant composition of one or more oxygenates. More particularly, the invention provides an improved blend control process for xBOB manufacture to maintain pre-determined properties of the oxygenate-containing finished gasoline from such a process.
2. Description of the Prior Art
Gasoline is comprised of a complex mixture of volatile hydrocarbons which are suitable for use as a fuel in a spark-ignition internal combustion engine, and it typically boils over a temperature range of about 80° to about 437° F. Although gasoline can consist of a single blendstock, such as the product from a refinery alkylation unit, it is usually comprised of a blend of several blendstocks. The blending of gasoline is a complex process, which typically involves the combination of from as few as three or four to as many as twelve or more different blendstocks to meet regulatory requirements and such other specifications as the manufacturer may select. Optimization of this blending process must take into account a plurality of characteristics of both the blendstocks and the resulting gasoline. Among others, such characteristics can include cost and various measurements of volatility, octane, boiling point characteristics, and chemical composition.
It is conventional practice in the industry to blend gasoline using blendstock ratios which are determined by mathematical algorithms also known as blending equations. Such blending equations are well known in the refining industry, and are either developed or tailored by each refiner and refinery for use in connection with available blendstocks. Blending equations typically relate the properties of a gasoline blend to the quantity of each blendstock in the blend and also to either the measured or anticipated properties of each blendstock in the blend.
Although hydrocarbons usually represent a major component of gasoline, it has been found that certain oxygen containing organic compounds can be advantageously included as gasoline components. These oxygen containing organic compounds are referred to as “oxygenate” or “oxygenates,” and are useful as components in gasoline because they are usually of high octane and can be a more economical source of gasoline octane than a high octane hydrocarbon blending component such as alkylate or reformate. As used herein, the term “oxygenate” includes both the singular “oxygenate” and the plural “oxygenates.” Current government regulations in the U.S. limits the oxygen content of gasoline to about 3.8 weight percent, based on elemental oxygen, and also requires that reformulated gasolines contain at least 1.5 weight percent of oxygenate or 10 volume percent denatured fuel ethanol, as in accordance with ASTM D4806-08b or the most current ASTM version. Oxygenates which have received substantial attention as gasoline blending agents include, but are not limited to, methanol, ethanol, tertiary-butyl alcohol, methyl tertiary-butyl ether, ethyl tertiary-butyl ether, and methyl tertiary-amyl ether. However, ethanol has become one of the most widely used oxygenates.
Oxygenate, if desired, usually is not blended into a gasoline at or within a refinery because oxygenates can be water soluble. As a consequence of this water solubility, an oxygenate-containing gasoline can undergo undesirable changes if an oxygenate-containing gasoline comes in contact with water during transport through any portion of a distribution system, which may include pipelines, stationary storage tanks, rail cars, tanker trucks, barges, ships and the like. For example, an oxygenate-containing gasoline can absorb or dissolve water which will then be present as an undesirable contaminant in the gasoline. Alternatively, water can extract oxygenate from the gasoline, thereby changing the chemical composition of the gasoline and negatively affecting the specifications of the gasoline. In order to avoid, as much as possible, any adverse effects from water, oxygenate-containing finished gasoline usually is manufactured by a multi-step process wherein the oxygenate is incorporated into the gasoline at a point which is near the end of the distribution system.
More specifically, gasoline which contains oxygenates generally is manufactured by producing an unfinished and substantially hydrocarbon blendstock, xBOB, at a refinery, transporting the xBOB to a product terminal in the geographic area where the finished gasoline is to be distributed, and mixing the xBOB with the desired amount of oxygenate at the product terminal. The combination of the xBOB with an oxygenate yields an oxygenate-containing finished gasoline which meets all regulations and specifications for sale.
As used herein, the substantially hydrocarbon blendstock, can be, and usually is, called an “xBOB” (Blendstock for Oxygenate Blending) when the blendstock is destined to be combined with a predetermined quantity and quality oxygenate to produce finished gasoline. xBOB is not a consistent blend and can vary with refinery or blending operations Examples of xBOB include, but are not limited to RBOB (reformulated blendstock for oxygenate blending), CBOB (conventional reformulated blendstock for oxygenate blending), CARBOB (California reformulated blendstock for oxygenate blending), Chicago BOB (Chicago RBOB or Chicago reformulated blendstock for oxygenate blending), Arizona RBOB, and Albuquerque RBOB. There can be a variety of other names for “BOB” gasolines.
Oxygenate-free finished gasoline can be manufactured within a refinery to very precisely fit the final US government specifications because analytical data for the product can be used to control the blending process. As a consequence, manufacturing costs are kept to a minimum by minimizing the amount of more costly refinery blendstocks in the blend.
When an xBOB is manufactured at a refinery, the xBOB properties are typically measured and controlled to meet intermediate specifications that differ from the finished gasoline. The intermediate specifications are developed to ensure that xBOB produced with a relatively wide range of compositions will always meet finished gasoline specifications after it is mixed with a predetermined quantity and quality oxygenate. As a result of targeting intermediate specifications, the xBOB and oxygenate mixture on average exceed the finished gasoline specifications. For example, an advanced closed loop feedback control system that is able to produce an xBOB to meet an intermediate octane target to within 0.01 octane points will often yield a finished octane after addition of ethanol that varies from 0.1 to 0.4 octane points above the minimum finished gasoline specification. Producing xBOB with lower precision in the meeting finished gasoline specifications after mixing the xBOB with oxygenate requires a more expensive average refinery blendstock and increases manufacturing costs.